The recent new environments for usage of disc drive memory systems have intensified design and performance needs including needs for heightened robustness. Besides traditional computing environments, disc drive memory systems are used more recently by devices including digital cameras, digital video recorders, laser printers, photo copiers, jukeboxes, video games and personal music players. Disc drive memory systems store digital information that is recorded on concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to store and retrieve information from the magnetic discs. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the stator. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the sleeve, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
Spindle motors have in the past used conventional ball bearings between the sleeve and the shaft. However, the demand for increased storage capacity and smaller disc drives has led to the design of higher recording area density such that the read/write heads are placed increasingly closer to the disc surface. A slight wobble or run-out in disc rotation can cause the disc to strike the read/write head, possibly damaging the disc drive and resulting in loss of data. Conventional ball bearings exhibit shortcomings in regard to these concerns. Imperfections in the raceways and ball bearing spheres result in vibrations. Also, resistance to mechanical shock and vibration is poor in the case of ball bearings, because of low damping. Vibrations and mechanical shock can result in misalignment between data tracks and the read/write transducer. These shortcomings limit the data track density and overall performance of the disc drive system. Because this rotational accuracy cannot be achieved using ball bearings, disc drives currently utilize a spindle motor having fluid dynamic bearings between a shaft and sleeve to support a hub and the disc for rotation. One alternative bearing design is a hydrodynamic bearing.
In a hydrodynamic bearing, a lubricating fluid such as gas or liquid or air provides a bearing surface between a fixed member and a rotating member of the disc drive. Hydrodynamic bearings eliminate mechanical contact vibration problems experienced by ball bearing systems. Further, hydrodynamic bearings can be scaled to smaller sizes whereas ball bearings have smallness limitations. However, hydrodynamic bearings suffer from sensitivity to external loads or mechanical shock events. Fluid can in some cases be jarred out of the bearing by vibration or shock events. Further, bearing fluid is susceptible to evaporation over time. Bearing fluids can give off vaporous components that could diffuse into a disc chamber. This vapor can transport particles such as material abraded from bearings or other components. These particles can deposit on the read/write heads and the surfaces of the discs, causing damage to the discs and the read/write heads as they pass over the discs. It is critical to avoid outgassing of contaminants into the sealed area of the head/disc housing.
Proper sealing is critical in the case of hydrodynamic bearings, and efforts have been made to address these problems. A capillary seal is typically employed to ensure fluid is maintained within a bearing. Here, a fluid meniscus is formed between two walls and capillary attraction retains the fluid. However, tests show that recent radial capillary seal designs fail at about 500 Gs of shock, and fluid leaks through fill holes at about 500 Gs of shock. Additionally, mobile applications require higher resilience to shock events than desktop or enterprise products. Laptop or portable computers can be subjected to large magnitudes of mechanical shock as a result of handling. It has become essential in the industry to require disc drives to be capable of withstanding substantial mechanical shock.
Fluid must be accurately filled into the journal gap and bearing. If excessive fluid is loaded into the bearing, the fluid will escape into the surrounding atmosphere landing on the surface of the disc and degrade the performance of the disc drive. If insufficient fluid is loaded into the bearing, then the physical bearing surfaces could contact, leading to increased wear and eventual failure of the bearing system.
Further, current oil fill and air evacuation methods for fluid dynamic bearings are relatively complex and costly due to the often awkward filling angles and tight clearances. It can be difficult to consistently accurately load fluid into the sharp corners of a shield hole. Further, current oil filling methods can leave a considerable amount of excess oil on the surfaces of the sleeve, which must be subsequently removed through an arduous post-cleaning process. The cleaning process can amount to ten percent of the total assembly cost of the motor.